Structured-light vision measurement is a measurement method based in taking images of measurement points or light stripes, which is formed by the intersection of a structured-light projected by a laser and the surface of an object being measured, by a camera, and then acquiring 3D information regarding the object surface using a laser triangular principle. According to a mode of laser projection, structured-light vision measurement may be divided into point structured-light, single line structured-light, multiple lines structured-light and circle structured-light, etc. Structured-light vision measurement has the advantage of a large scale of measurement, noncontact, high speed, good system flexibility and moderate precision, which is widely used in the areas of reverse engineering and products online testing, etc.
The key to successful application of structured-light vision measurement is the calibration of structured-light vision sensor, which comprises calibration of the camera's intrinsic parameters and structure parameters, wherein the calibration method of the camera's intrinsic parameters is not introduced here because it is relatively mature. So far, as to the calibration for the structure parameters of the structured-light vision sensor, many scholars have researched the subject and presented some new methods such as a fiber drawing method, sawtooth target method and a cross ratio invariability method based on the 3D target, etc.
To be more specific, the so called “fiber drawing method” lets the structured-light project on some spatial distributed fibers which are non-coplanar. Because of the dispersion of the fibers, bright points are formed on the fibers and imaged on the image plane. The spatial coordinates of the bright points may be measured by electronic theodolite, thereby permitting solving of the position parameters between the structured-light and the camera by the bright points' coordinates in the image plane and spatial. This calibration method needs two electronic theodolites to measure the spatial coordinates of the bright points. If more calibration points are acquired, multiple times of manual aim are needed, of which the operations are complicated.
When it comes to the sawtooth target method, an article entitled “A New Structure Parameters Calibration Method of Structured-light Vision Sensor” and published in the Chinese Journal of Scientific Instrument, 2000, 21(1):108-110 by Fajie Duan et al. presents a structure parameters calibration method of a structured-light vision sensor according to the features of structured-light vision sensor, in which a simple calibration target and a 1D bench are used to realize the highly precise calibration of a line structured-light sensor. In this method, no other auxiliary apparatus is needed to measure the coordinates of the points on the light plane. However, the operations of the method are complicated because the attitude of the 1D bench or the structured-light sensor should be adjusted in order to make the light plane perpendicular to the edge line. Moreover, the cost of processing a sawtooth target is high, while the sawtooth edge line is limited and the calibration points that can be acquired are less.
Concerning the cross ratio invariability method based on a planar target, it is a calibration method for the structure parameters of a structured-light vision sensor. The cross ratio invariablilty method is described in an article entitled “Complete Calibration of a Structured-light Stripe Vision Sensor Through Planar Target of Unknown Orientations[J], Image and Vision Computing, Volume 23, Issue 1, January 2005, Pages 59-67)” by Fuqiang Zhou. This method uses the cross ratio invariability principle to acquire a calibration point, the lines of which on the target are few, usually 3 to 10 lines. Only one calibration point can be found in a line of feature points (at least three). Therefore, using this method, 3 to 10 calibration points from the target at one position may be obtained. This method requires no auxiliary apparatus, and there is no occlusion problem and the operations are simple. That is why the method is only suitable for line structured-light calibration, and there will be a large fitting error when the method is used to calibrate non-line structured-light.
As to the rapid calibration method of the line structured-light sensor based on a coplane reference, this calibration method is for the structure parameters of a structured-light vision sensor, as described in a Chinese patent by Jigui Zhu, entitled, “Rapid Calibration Method of Line Structured-light Sensor Based on Coplane Reference”, the patent number of which is 200510013231.7. The method calibrates a line structured-light based on an intersection line of the coplane reference and the plane determined by the optical center of the camera and the images of the line light stripes. This method requires no auxiliary apparatus, and there is no occlusion problem and the operations are simple, whereas, it can only be used in the calibration of a line structured-light or a multiple lines structured-light.
According to the analysis above, the fiber drawing method requires electronic theodolites as an auxiliary apparatus and multiple times of manual aim which is complicated to operate. A few calibration points may be acquired in the sawtooth target method because the sawtooth edges are limited, but the cost of processing the sawtooth target is high. The scope of the calibration method of cross ratio invariability based on planar target and rapid calibration method based on coplane reference for line structured-light sensor is small. The two methods are only suitable for calibration of line structured-light or multiple lines structured-light.